14.2.6.4 Physical Structure of an InnoDB Index

All InnoDB indexes are
B-trees where the index
records are stored in the leaf pages of the tree. The default
size of an index page is 16KB.

When new records are inserted into an InnoDBclustered index,
InnoDB tries to leave 1/16 of the page free
for future insertions and updates of the index records. If index
records are inserted in a sequential order (ascending or
descending), the resulting index pages are about 15/16 full. If
records are inserted in a random order, the pages are from 1/2
to 15/16 full. If the fill
factor of an index page drops below 1/2,
InnoDB tries to contract the index tree to
free the page.

You can configure the page
size for all InnoDB tablespaces in a
MySQL instance by setting the
innodb_page_size configuration
option before creating the instance. Once the page size for an
instance is set, you cannot change it. Supported sizes are 16KB,
8KB, and 4KB, corresponding to the option values
16k, 8k, and
4k.

A MySQL instance using a particular InnoDB
page size cannot use data files or log files from an instance
that uses a different page size.